The present invention relates to an oil supply system for an engine, specifically, an oil supply system for an engine which is capable of reducing engine noise by securing lubricating oil films of drive force transmission system members (e.g., a piston pin and a connecting rod) for transmitting a drive force due to combustion, when an engine load is low (e.g., during an idle operation).
Conventionally, oil supply systems for supplying engine oil (hereinafter, simply referred to as “oil”) to various parts of an engine mounted on a vehicle (e.g., automobile) are well known. For example, JP2014-159757A (particularly paragraphs [0010]-[0013]) discloses an art of controlling a variable displacement oil pump to adjust, to a target oil pressure, a required oil pressure of a device using oil as operational oil, when a required oil pressure of the device is higher than that of a device using the oil as cooling oil and is also higher than that of a device using the oil as lubricating oil and cooling oil. Thus, a drive force of an oil pump is suppressed to a necessary minimum level and a fuel consumption is improved, while satisfying the required oil pressures of all the devices.
However, by adjusting a discharge amount (further discharge pressure) of the variable displacement oil pump according to the required oil pressures of the devices as in JP2014-159757A, for example, when an engine load is low (e.g., during an idle operation) in a warming-up operation state, engine noise may increase for the following reasons.
First, sliding surfaces of drive force transmission system members (e.g., a pin boss of a piston, a piston pin, and a smaller end part of a connecting rod) for transmitting a drive force due to combustion are lubricated by an oil mist. Specifically, for example, oil leaked from, for example, a bearing metal of a crankshaft bearing rotatably supporting a crankshaft is spread and atomized (becomes a mist) by rotation of the crankshaft, and becomes an oil mist flowing inside a crank case. The oil mist permeates and attaches to the sliding surfaces of the drive force transmission system members and lubricating oil films are formed thereon. Thus, a to-be-lubricated surface (sliding surface) is lubricated by an atmosphere with an oil mist even without being supplied with lubricating oil from the oil pump. Such lubrication is referred to as “atmospheric lubrication.”
On the other hand, after the engine is warmed up, the discharge amount of the variable displacement oil pump is adjusted to achieve the required oil pressure according to an operating state of the engine, and the drive force of the oil pump is suppressed to the necessary minimum level. In other words, an oil pressure controller is provided, which controls the variable displacement oil pump to adjust the discharge pressure of the variable displacement oil pump, which is capable of adjusting the discharge amount of oil, to be the target oil pressure according to the operating state. In this case, the required oil pressure, further the target oil pressure, is set lower as the engine load becomes lower. Therefore, when the engine load is low (e.g., during the idle operation) in the warming-up operation state, the discharge amount of the oil pump is reduced, which causes reduction of the generation amount of oil mist. As a result, the atmospheric lubrication by the oil mist, in other words, the formation of the lubricating oil films on the sliding surfaces of the drive force transmission system members or a formation of a lubricating oil film between sliding surfaces of a piston skirt and a cylinder liner, etc., becomes insufficient and causes increased engine noise when the piston operates.